The present invention relates to a thermal printer and more particularly to a thermal printer that is arranged so as to eliminate unevenness of hardness of covering rubber constituting a platen roller thereby preventing unevenness of an image derived from this unevenness of hardness or the like.
Image recording (also referred to as "thermal image recording") using a thermal recording material such as a thermal film or the like is utilized in recording an image produced in diagnosis by ultrasonic scanning or the like. The above-described thermal image recording eliminates the need for wet processing and offers several advantages including convenience of handling. Hence, in recent years, the practical use of the thermal image recording is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnosis such as MRI (magnetic resonance imaging), X-ray photography and the like where images of large size are required is also under way.
As is known, the thermal image recording involves the use of a thermal head in which heat-generating elements for heating the thermal recording material imagewise so as to record the image are arranged in one direction; and the thermal head and the thermal recording material are relatively transported in a direction perpendicular to the direction in which the heat-generating elements extend thereby performing the image recording. In the thermal image recording, energy is applied to respective heat-generating elements of the thermal head based on image signals to heat them imagewise thereby accomplishing image recording.
As is also known, thermal image recording is basically categorized into two types, i.e., a direct recording (direct print) system for recording an image using a monochromatic or multicolor thermal color-forming material and a transfer recording system for performing monochromatic or multicolor transfer recording using an ink ribbon or the like. Various types of specific methods are put into practice in respective recording systems.
For the thermal image recording, in any of the direct thermal recording and transfer recording systems, the thermal head, particularly, a glaze of a recording section, is pressed against the thermal recording material on the platen (platen roller). In this case, it is without saying that enhancing the accuracy of a relative position between the glaze of the thermal head and the platen is necessary. As a reference of the above, for example, description of Unexamined Published Japanese Patent Application (Kokai) No. 295440/1997 can be referred to.
Besides, in the practical thermal image recording (hereinafter referred to simply as "image recording"), various characteristics of covering rubber of the platen (the platen roller), namely, material quality, physical properties, sizes and interrelations therebetween are important. In case they are not appropriate, even if the accuracy of the relative position between the glaze of the thermal head and the platen roller has been enhanced, image defects in various forms of unevenness may appear on the recorded images obtained as a result of the image recording. These forms of unevenness, blurs of images and the like were influential factors which have decreased the qualities of images recorded with the thermal printer.
As is described in detail in Unexamined Published Japanese Patent Application (Kokai) No. 76697/1998 "Uneven density compensation method of thermal recording apparatus", it is known that uneven density derived from recording density is generated in a border where recorded density changes greatly. This type of density unevenness is generated in the following steps:
For example, when the recorded image as shown in FIG. 7 is formed, friction between the thermal recording material and the thermal head varies in accordance with density to be recorded so that torque of a transport motor necessary for transporting the thermal recording material varies greatly.
Next, in accordance with changes of the torque, a shape of the platen roller varies; and the larger the change of the torque the larger the deformation of the platen roller. When the transport torque changes from a smaller side to a larger side, the platen roller is rapidly deformed whereas when the transport torque changes from a larger side to a smaller side, the deformed platen roller tends to rapidly recover its original form. Therefore, in the recorded image illustrated in FIG. 7, when an edge part where a lower density portion in a lower side of the figure is shifted into a high density portion in the middle of the figure is recorded, the transport torque is rapidly decreased, the greatly deformed platen roller goes back to the original state and a transport rate of the thermal recording material becomes fast for a moment and recording density is decreased; hence, a white streak is generated in a traverse direction in the figure.
On the other hand, in the recorded image illustrated in FIG. 7, when a border where the high density portion in the middle of the figure is shifted into a low density portion in an upper portion of the figure is recorded, the transport torque is rapidly increased, the platen roller is greatly deformed and the transport rate of the thermal recording material becomes slower for a moment and the recording density is increased; hence, a black streak is generated in the traverse direction in the figure.
Such a traverse streak derived from a large density change of the recorded image deteriorates the quality of the recorded image so that elimination of such traverse streak, as well as elimination of the image quality deterioration (decrease of clarity and generation of a blur) derived from the fluctuation of the pressing force between the thermal head and the platen roller, has been desired.